Fix crashes related to WCT for ARC T

- Add two more null checks as we already do in other places in
 the class.
- Don't let Task#setDragResizing() throw an exception based on
 win mode as the win mode Shell is seeing can be different from
 that in Core depending on the timing.

Bug: 294163171
Bug: 294163886
Bug: 294162591
Test: WM CTS
Change-Id: I207f5a5c9301ccebe7311b72c93cf54f98fe6933